1. Field of the Invention
The present invention relates to a focus detecting apparatus for detecting a focus state of a taking (or photographing) lens of, for example, a camera, with respect to an object to be photographed.
2. Description of Related Art
In a known focus detecting apparatus of a single lens reflex camera or the like, bundles of rays transmitted through different portions of an exit pupil of a taking lens are converged onto a pair of line sensors by an image reforming lens of a focus detecting optical system, so that the focus state of the taking lens can be detected in accordance with a relationship between outputs of the line sensors. The principle of focus detection by the focus detecting apparatus is disclosed, for example, in U.S. Pat. No. 4,636,624. A known apparatus for detecting the focus state of a taking lens with respect to an object located at a position other than the center of an image plane has been disclosed in which a detecting area of the focus detecting apparatus is deviated from the optical axis. However, as the distance of the off-axis detecting area from the optical axis increases, vignetting of the focus detecting optical system pupil occurs when the position or size of the exit pupil of the taking lens changes as a result of a lens change (e.g., when using an interchangeable lens) or during a zooming operation. In such cases, the area of the focus detecting optical system pupil, through which the bundle of rays is transmitted, is reduced. As a result, the applicability of such an automatic focus detecting system to an interchangeable lens is restricted. It is also possible that the automatic focus detecting system will not work at a specific focal length within a zooming range.
To solve these problems, an application, commonly assigned with the present application, has been filed under the serial number of U.S. Ser. No. 07/885,369. According to this application, a focus detecting apparatus is provided in which a pair of bundles of rays transmitted through different portions of an exit pupil of a taking lens are transmitted through a common AF detecting zone formed on an imaginary focal plane of the taking lens, so that images of the bundles of rays are formed at predetermined positions. The focus detecting apparatus includes at least two image reforming optical systems which reform images of the bundles of rays incident on the AF detecting zone from different directions, in accordance with a change in position of the exit pupil in an optical axis direction of the taking lens. The apparatus further includes at least one pair of line sensors located at the predetermined positions, so that the images of the bundles of rays are reformed on the respective line sensors through the respective image reforming optical systems.